• Calibration of Digital Hygrometers for Vivaria Use

    This popped into my head after participating in a recent thread on here. I figure it will be useful to quite a few folks. Yes, this is the method I use.

    Calibration of Hygrometers for Vivaria Use

    Many reptile and amphibian keepers use hygrometers, both digital and analog, to help maintain a healthy environment for their pets. A hygrometer is a gauge type device to measure relative humidity in the ambient atmosphere. (In English that means it tells you how much water is in the air, sort of...) What many hobbyists do not realize is that these devices should be calibrated regularly or the readings become highly inaccurate over time. This can lead to all sorts of issues, from mold out breaks to dead, desiccated pets. I do it every six months or so.

    <DISCLAIMER> This is not intended as a laboratory guide. This method is perfectly suitable for amphibian keepers. Also note that this should be done at ambient room temperature at 68-70F or roughly 20C)


    Materials Needed:


    Air tight plastic container such as a small zipper seal sandwich bag or Tupperware type container. Try to use a container as small as possible for better results.

    Plastic bottle top from a two liter soda-pop bottle. (20oz and 1 liter tops are the same size as well!)

    Table Salt. Yes, regular plain old fine grain table salt. Rock salt and sea salt work, but generally have larger grain size making it hard to work with.

    Hygrometer(s)

    Measuring Teaspoon.
    For folks in the US any measuring spoon of this size will work. Do not try to use an actual tea spoon as the volume is a bit larger. For the rest of the world that bothered to get with the times and go metric (bear with me here, I don't cook in metric!) use roughly 5.7g of salt. (By volume I believe this is about 5mL , but I have no idea what a metric measuring spoon is graduated with!)

    Eye dropper or pipette or anything else that allows you to control measure small amounts of liquids. (I use a graduated plastic syringe intended for dispensing wood glue...)


    You should only calibrate one hygrometer at a time using this method.

    Now that you have your shopping list, the rest is easy. Take the bottle cap and measure in one US Teaspoon of plain old table salt. Carefully add regular tap water a few drops at a time until the salt is saturated. The salt should be a thick, wet paste. All the water should be absorbed by the salt. If you have free standing water in the cap, you added too much water! (I am very lazy so I add water, then soak up the extra with a quick dab from a dry paper towel.)

    Next, place the salt paste cap and your hygrometer in the air tight container and seal it up. Now go clean your enclosures, do water changes, and feed your critters. Watch a few videos on the web. Take your pug for a walk. Anything to burn up a few hours. Yell at the teenagers walking through your expensive landscaping... At the bare minimum you need to let it sit for two hours. Longer is better. (I let mine sit for 12 hours.)


    After you have let your hygrometer sit, check the reading. At 70F(21.1C) the hygrometer should read 75% humidity. If it does not, follow the manufacturer's instructions and push buttons so it does. If you are using an analog Hygrometer, there is an adjustment screw on the back to set the indicator. If your hygrometer cannot be set for what ever reason, replace it with one that can.


    Your hygrometer is now ready to go back into the enclosure. I use the salt for my hot pretzel snacks.


    This procedure can be found all over the web. There are all sorts of versions of it. I will not bother to provide references as I learned it originally from the Boy Scouts years ago while earning my Environmental Science and Weather merit badges.
    This article was originally published in forum thread: Calibration of Digital Hygrometers for Vivaria Use started by SludgeMunkey View original post
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